Answer: The challenge with passive solar structures like cold frames and a shed like yours is moderating extreme temperatures. During the day, a south-facing window can collect a lot of heat, and cool down dramatically at night. If the only thing you plan to grow in your shed is seedlings, then it'll probably be warm enough if you insulate the walls and use bricks as a heat sink. Although you don't plan to have electricity at the shed, it would help to have heat in the root zone in the form of heating cables or mats, since for most seeds, the soil temp should remain around 65-70F for germination and good growth. Temperature-sensitive vent openers (hydraulic, not electric) are a good option for greenhouses. When the interior gets hot, they automatically open vents/windows to let in cool air, and close when the interior cools down.
As long as there is adequate air circulation, fuel fumes shouldn't be a problem. However, if the shed smells very strongly of fuel, especially during the heat of the day, then remove power equipment/fuel cans from the shed until you're done with seedstarting. Or perhaps you can wall off part of the shed for seedstarting, and keep equipment in a more ventilated are of the structure.
Here are a couple of catalogs that offer hobby greenhouse supplies:
Charley's Greenhouse Supplies, 17979 State Rt. 536, Mt. Vernon, WA 98273-3269, http://www.charleysgreenhouse.com, 800/322-4707
Gardener's Supply Co., 128 Intervale Rd., Burlington, VT 05401, http://www.gardeners.com, 800/863-1700.
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